In situ measurements of target strength were made of krill (Euphausia
superba) and salps (Salpa thompsoni) at 120 and 200 kHz. Concurrently, a 2-m
Isaacs-Kidd underwater trawl was used to sample the zooplankton population;
animal length, wet weight, and maturity stages were recorded. These data were
combined to derive empirical models of TS versus length for both species. The
individual target strength measurements were collected at 120 kHz with a
split-beam echosounder and a single-target detection algorithm. Because the two
transducers were essentially colocated, range bin and off-axis angles from the
120-kHz detections were used to extract the corresponding target strength from
the 200-kHz single-beam data. Backscattering strengths of salps are shown to be
higher at 200 than 120 kHz. Utilizing these scattering characteristics, signal
separation methods are explored that would be useful in acoustic surveys of
krill distribution and abundance. Data were collected as part of the United
States Antarctic Marine Living Resources Program, near Elephant Island,
Antarctica, during the Austral summer of 1994. [Work of D. A. Demer was
supported by the Hertz Foundation.]